It is known to generate electrical power on-board floating barges located offshore and to transfer the power to shore and into the electricity network. Normally, such barges are spread-moored in a manner which does not allow the barge to weathervane around the mooring in response to the action of wind, waves and currents. Alternatively, such barges may be turret-moored wherein mooring lines are attached to a turret rotatably mounted in the hull of the barge. This system allows the vessel to weathervane around the turret and the mooring lines in response to wind, wave and current action.
If a barge is turret-moored in this way, an electrical swivel apparatus is required on top of the turret in order to transfer the power from the weathervaning barge to stationary subsea cabling. Usually, a large amount of power is being produced in these situations, in the order of 100 to 500 MW, and thus a large electrical swivel is required.
Furthermore, offshore transmission of large amounts of AC electric power over long distances requires very high voltages in order to avoid the need for large cross-section copper cables and by consequence very heavy and expensive cables.
A solution to reduce the cross section of the conductor in submarine cables is to transform the electric AC power into DC power, but this requires big, expensive, and relatively inefficient converter stations.
A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance distribution, HVDC systems require cables with smaller cross-section and therefore are less expensive and suffer lower electrical losses.
For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful.
The actual high voltage electric swivel for single point moored floating production units is a bottleneck for the increase of the voltage of the submarine cables. Very high voltage swivels made according to actual technologies would require a very big spacing between the slip rings, and very high quality insulating fluid. Furthermore, a very high voltage electric swivel would require big and expensive transformers on the floating production unit.
In WO 2009/128724 a rotary transformer is described for use in the turret of offshore floating production storage and offloading (FPSO) systems or on wind turbines. In FIG. 10 of WO 2009/128724 a three-phase rotary transformer with a vertical air gap is described in which the inner and outer swivel rings are provided with radial legs each carrying a primary or secondary conductor winding, which windings are facing one another. The known swivel requires a relatively large size for high power applications. Furthermore, the distribution of the windings on the outer rotor part results in a relatively inefficient coupling of the magnetic field that is generated in the windings of the inner stator core element. The magnetic field traverses a relatively large air gap to induce an induction current in the coils on the outer ring-shaped rotor.
It is therefore an object of the present invention to provide a power swivel which allows efficient power transfer. It is a further object to provide a power swivel that can transfer high voltages while being of a compact design and of a relatively low weight.
Furthermore, the power swivel according to the present invention should allow for adapting the power to needs (either power consumption or production) of a floating unit in dependence on the power levels and transmission distances while minimizing power losses. The power swivel should be able to avoid the use of dielectric fluid as a main insulator, thus reducing the risk of a short circuits in the swivel and also avoiding any wear caused by sliding contacts. The present invention aims to propose a much more compact and more efficient alternative to HVDC technology for transmission of high power over medium to long distances.